The present invention relates to a process for making in-situ Silicon Carbide in the form of particulate, whiskers and fibers in an inorganic composite matrix.
The process of the present invention particularly relates to the use of wide variety of natural fibres such as jute, hem, sisal or any other natural fibre having cellulosic or hemicellulosic fibre as its backbone as raw materials for providing useful carbon for the formation of silicon carbide in the form of particulate, whiskers and fibres in an inorganic composite matrix selected from Silicon Carbide-carbon and carbon matrix.
The main usage of the silicon carbide particulate, whiskers, fibre in Silicon Carbide-carbon or carbon composite matrix is in the field of engineering materials in any shape as may be deemed fit.
The present day method of making Silicon Carbide particulate, whisker and fibre reinforced composite in Silicon Carbide-carbon matrix or carbon matrix essentially consists of seeding graphite substrate with metal droplets such as Fe, Co, Cr and Mn as catalyst for the whisker formation. Methane and silicon monoxide supply C and Si respectivelyxe2x80x94the references for which may be made to xe2x80x9cSynthesis and Characterization of VLSxe2x80x94Derived Sic whiskersxe2x80x9d of P. D. Shaiek in Conf. Whisker and Fibre Toughened Ceramics, Oak Ridge T N (1988) and to xe2x80x9cReview of VLS Sic Whisker Growth Technologyxe2x80x9d by W. E. Holler and J. J. Kim in Ceram. Engg. Sci. Proc., vol. 12, pp979-991 (1991) or making fibre by melt extrusion and suspension spinning of compositions of ultrafine SiC powders and organic additives such as polyvinyl butyral respectively followed by sintering or making fibre by melt spinning polymers which can be rapidly cured in the solid state and polymerised to ceramic fibres with compositions which are stoichiometric silicon carbide or which are carbon-rich or silicon-rich silicon carbide the reference for which may be made to xe2x80x9cSilicon Carbide: from Acheson""s Invention to New Industrial Productsxe2x80x9d by W. D. G. Boecker in cfi/Ber. DKG74, (5), 1997. The fibres and whiskers produced by the above processes are mixed mechanically with the matrix material and are fabricated into different sizes and shapes followed by heat treatment at different temperatures for consolidations the references for which may be made to xe2x80x9cPressureless Sintering of Al2O3./SiC Materials: Effect of the xe2x80x9cReducing Atmospherexe2x80x9d by G. Urretavizcaya, J. M. Porto Lopez and A. L. Cavalieri, J. Eu. Ceram. Soc. 17 1555-63 (1997). In a process inorganic polymers that are ceramic precursors are spun into fibres by melt-spinning or solventxe2x80x94assisted dry spinning, stabilising the fibres to prevent remelting followed by thermally decomposing into fibres, the references for which may be made to German P.2,618150; French P.2,308,590; Japanese P.51 130325, 51 139929, 51 147623 (1976). In slurry spinning a dispersion of crystalline ceramic particulate in a carrier fluid is formed into a fibre, converted to fibre by thermal conversion by several heating stages that may include passing the fibre through a flame. The process generates particulate of not more than 1 um to control shrinkage the references for which may be made to E.I. du Pont de Nemovrs and Co. B. P. 1,264,973 (1972); U.S. Pat. No. 3,808,015 (1974); U.S. Pat. No. 4,753,904 (1988), Mitsul Minie Co. Ltd. Japanese P 217182. (1986); European P. 0,206,868xe2x80x94A2 (1988), U.S. Pat. No. 4,812 271 (1989).
The overall process has several drawbacks that may be listed below:
1. Number of steps involved in the overall process is higher.
2. Handling of whiskers and short fibres require special arrangements.
3. It is difficult to disperse whiskers and short fibres uniformly in the matrix.
4. Silicon Carbide whiskers particularly of aspect ratio less than 10 cause health hazard.
A principal object of the present invention is to provide a process for making in-situ silicon carbide in the form of particulate, whiskers and fibres in an inorganic composite matrix selected from Silicon Carbide carbon and carbon matrix composite which obviates the drawbacks as detailed above.
Yet another object of the present invention is to utilise natural fibres of plant source.
Still another object of the present invention is to reduce the total number of unit processes in the overall operation.
Yet another object of the present invention is to form whiskers and fibres in-situ during processing to eliminate totally the possibility of health hazard.
Accordingly the present invention provides a process and composition useful for making in-situ silicon carbide in the form of particulate, whiskers and fibres in an inorganic composite matrix selected from silicon carbide carbon and carbon composite matrix, wherein the said composition comprises:
In an embodiment of the present invention the inorganic composite matrix such as a silicon carbide-carbon composite matrix further comprises:
In yet another embodiment the inorganic composite matrix such as a carbon composite matrix further comprises:
In yet another embodiment the fibre used is a natural fibre selected from the group consisting of jute, sisal, hem and any other natural fibre having cellulosic or hemicellulosic constituent at its backbone.
In yet another embodiment the curing agent used is selected from hexamine, para toluenesulphonic acid and para formaldehyde most preferably hexamine.
In yet another embodiment the molecular weight of phenolic resin used is in the range of 450-700.
In still another embodiment the organic solvent used is selected from methanol, toluene and benzene.
In another embodiment, the present invention provides a process for making in-situ carbide in the form of particulate, whiskers and fibres in an inorganic composite matrix, which comprises, dissolving 38 to 73.5 Wt % of phenolic resin in -an organic solvent to obtain a phenolic resing solution, adding 4.2-11.0 Wt % of a curing agent optionally adding 9.4-12 Wt % of silicon carbide powder to obtain a resinxe2x80x94mix followed by impregnating 1.66.5 wt % of natural fibre-with the said resin mix dried at 60xc2x0-70xc2x0 C. for a period in the range of 1-5 hrs. to obtain a dough in the form of a composite plate sheet, drying the said composite plate or sheet at a temperature in the range of 70xc2x0-90xc2x0 C. for a time period in the range of 1-2 hrs., drying the composite plate or sheet, heat treating the dried composite plate or sheet at a temperature in the range of 150xc2x0-200xc2x0 C. for a period in the range of 1-2.5 hrs., impregnating the resultant composite plate or sheet with 10.4-42 Wt % tetraethyl orthosilicate in vacuum, subjecting the impregnated composite plate/sheet to heat treatment in absence of air initially at a heating rate in the range of 2xc2x0-5xc2x0 C. per minute up to a temperature in the range of 200xc2x0-400xc2x0 C. followed by further heating at the rate of 10xc2x0-15xc2x0 C. per minute up to a temperature in the range of 1400xc2x0 C.-1850xc2x0 C., maintaining the final temperature for a period in the range of 0.5-2 hrs.
In yet another embodiment the natural fibre introduced in the body in desired alignment is selected from unidirectional, multidirectional, woven, and randomly oriented structure.
In yet another embodiment the matrix material used is of different particle dimension ranging from nano to micron size.
In still another embodiment the heat treatment is done in an inert atmosphere or in vacuum.
In another embodiment the composition is useful for making in situ silicon carbide in the form of particulate, whiskers and fibres in an inorganic matrix selected from silicon carbide-carbon and carbon composite matrix.
The process of the present invention can be used to produce in-situ silicon carbide in the form of particulate, whiskers and fibres in an inorganic composite selected from Silicon Carbide-carbon and carbon composite matrix of various shapes and sizes required for application as engineering components. Natural fibres of plant origin contain all the ingredients of forming silicon carbide if sufficient amount of silicon is provided. Organo silicon compounds such as ethyl silicates give active silica on pyrolysis. This active silica inturn reacts with carbon formed from cellulosic materials like fibres of plant origin. Silicon carbide thus formed is in molecular dimension and with proper catalyst and seeding crystals, unidirectional grain growth occurs to form long fibres of small diameters or whiskers of different aspect ratios. Isolated silicon carbide grains may grow in the three dimensions resulting in particulate formation. If this complex set of network structures is allowed to grow in an inorganic composite matrix selected from Silicon Carbide carbon and carbon matrix the resulting microstructure consists of randomly orientated fibres and whiskers in a homogeneous matrix leading to enhanced fracture toughness imparting some machinability in otherwise non-machinable material.